This invention relates generally to diode pumped gas lasers that are end-pumped. More particularly, the present invention relates to the use of modern semiconductor laser diode sources to optically pump gas atoms or molecules that can produce visible, near-infrared (NIR), mid-infrared (MIR), and far-infrared (FIR) laser transitions.
The definition of "gas" includes a single gas or a mixture of atomic and/or molecular gases, and atomic and molecular vapors. It has been known for some time that triatomic molecules, for example, can be optically rather than electrically-pumped. Because the diode wavelengths needed to demonstrate efficient optical pumping of CO.sub.2 and N.sub.2 O fall in the NIR to MIR range, such sources have not heretofore been available. High power laser diodes available today have wavelengths ranging from about 635 nm in the red to about 1.6 .mu.m in the NIR, although lower power semiconductor diodes can now be obtained commercially out to about 2 .mu.m.
Recently a new type of semiconductor diode laser known as the quantum cascade diode laser has been demonstrated with output in the 3-15 .mu.m range which is ideal for pumping a number of triatomic molecule based lasers including CO.sub.2 and N.sub.2 O. NIR emitting diode lasers can also be used to optically pump a number of diatomic molecules, including HF, DF, HCl, HBr, and CO. Recently developed GaN semiconductor diode lasers have extended the emission range down to just above 400 nm in the near-ultraviolet or blue spectral region, making possible the demonstration of visible and NIR laser emission from atomic or molecular gases. Optical pumping of molecular iodine vapor has been demonstrated where pumping at 493-501 nm resulted in emission at 1.315 .mu.m; this system for example is amenable to pumping using a semiconductor laser diode or diode array.
Semiconductor diode-pumping of atomic and molecular gas lasers can produce a number of substantial benefits, including producing new laser transitions that heretofore could not be achieved by pumping with electrical discharges, higher laser efficiency as compared to electrical discharge lasers, and substantially less heating of the lasing media than that obtained with electrical discharges. Other substantial advantages include the use of low-voltages rather than the lethal high-voltages associated with present electrical discharge lasers, the elimination of gas dissociation which limits the lifetime of present gas lasers, reduced electric noise, and the elimination of discharge instabilities which lead to fluctuations in the laser output. Also, because there are no electrical discharge imposed limitations to the partial pressures involved, higher gain/cm can be achieved. Reduced heating of the gas will also improve the gain/cm. Because the large power supplies and associated equipment used for electrical discharges can be eliminated with diode-pumping, more compact laser sources will result. Because of the elimination of discharge instabilities and the precision with which the output of diode lasers can be regulated, very low-noise stable diode-pumped gas lasers can be demonstrated.
A substance is optically-pumped if it has an optical absorption band that absorbs a fraction or all of the incident optical radiation whose wavelength or bandwidth center is completely or partially overlapping with the absorption band of the substance. In one method, the absorption of optical photons had the effect of raising ground state or excited state electrons in the substance to an excited state or higher excited state, respectively, for the purpose of achieving a population inversion in the substance. In another method, the absorption of photons is used to excite or populate an excited state in a first atom or molecule and the excitation is then transferred to a second atom or molecule whose upper laser level is near or below the energy level in the first atom or molecule. In yet another method, the absorption of a photon is used to photo-dissociate a first molecule into excited atoms with a populated upper laser level. Such a population inversion is well known to those familiar with the state-of-the-art of laser technology. In general, the number of excited ions/cm.sup.3 or excitation density R.sub.e (r,z) can be calculated from ##EQU1##
where .lambda..sub.p is the pump or diode wavelength, h Planck's constant, c the speed of light, l(r, z, t) the incident diode intensity which in general is a function of the radial r and azimuthal (propagation direction) z coordinates and time t. .alpha..sub.p is the monochromatic absorption coefficient, used in calculating the absorption of the intensity according to EQU l(r,z,t)=l.sub.0 (r.sub.r t)exp(-.alpha..sub.p z).
The absorption coefficient is calculated from EQU .alpha..sub.p (.lambda..sub.p)=.sigma..sub.p (.lambda..sub.p)n.sub.D
here .sigma..sub.p (.lambda..sub.p) is the pump absorption cross-section which is a function of the pump wavelength, and n.sub.D is the absorbing species density. Note that the above equations are for an assumed monochromatic pumping or diode source; in cases where the source has an appreciable bandwidth the equations can easily be suitably modified. It is important to note that the equation in line 11 predicts that the pump light, and therefore the excitation density according to the equation in line 6, vary exponentially with penetration distance into the absorbing gas medium. This of course also means that the inversion density and gain also vary exponentially with distance. For end-pumped diode-pumped gas lasers, in analogy with diode-pumped solid-state lasers, the inversion and gain vary substantially with penetration distance and thus the obtained profiles are very inhomogeneous. This is in contrast to the pumping methods disclosed by Meinzer (U.S. Pat. Nos. 5,506,857 and 5,379,315) which disclose a "homogeneous excited medium" (U.S. Pat. No. 5,506,857, claim 1, line 17) or "said pumping light being injected relatively uniformly into said gas." (U.S. Pat. No. 5,379,315, claim 1, Line 13) In addition, since the diode-pump sources from single diodes, diode-arrays, or from diodes coupled through fibers often vary transversely with a Gaussian or Gaussian-like distribution, the resulting inversion density and gain in the diode-pumped gas laser will also show strong variation with both the transverse and distance (longitudinal) coordinate. Again in analogy to diode-pumped solid-state lasers, this feature makes it possible to pump directly into the fundamental TEM.sub.00 Gaussian resonator mode and achieve single transverse mode output with very good overlap between the pump and the resonator modes. A large overlap efficiency results in large extraction and overall laser efficiency. For direct excitation, and in the absence of lasing, the absorption of diode light produces an upper level inversion density n.sub.m whose variation with time t is given by the equation ##EQU2##
where .tau..sub.F is the upper level fluorescence lifetime and all other processes that depopulate the upper laser level, such as collisional de-excitation, are lumped into a single quenching constant k.sub.Q. The lower laser or terminal laser level in general has a population N.sub.T that is, in the case of four-level lasers or quasi-four level lasers, insignificant as compared to the upper laser level. The primary reason for this is that the lifetime of the terminal level, .tau..sub.T, is significantly less than the fluorescence or effective lifetime of the upper laser level. The gain/cm g.sub.0 of the laser transition is given most generally by EQU g.sub.0 =.sigma..sub.e (n.sub.m -(g.sub.2 /g.sub.1)n.sub.T)
here .sigma..sub.e is the transition effective stimulated-emission coefficient and g.sub.2 and g.sub.1 are the upper and lower laser level degeneracy's respectively. To achieve net positive gain, one must then have EQU n.sub.m &gt;(g.sub.2 /g.sub.1)n.sub.T
Once such a population inversion is achieved and is able to overcome the laser resonator threshold, power in the form of a laser output beam can be extracted from the substance using optical resonators. Optical pumping may be implemented by the use of incoherent sources, for example flash lamps, the sun, and various types of explosives, or coherent sources, for example lasers (i.e. a laser-pumped laser) and semiconductor diode lasers. In some instances optical pumping sources are partially coherent as is obtained with a laser diode array whose individual emitters are coherent but whose total output is partially coherent or completely incoherent.
The use of optical-pumping to create a population inversion in a lasing medium is well-established. Flash lamps and diode lasers have both been successfully used to create population inversions and demonstrate laser output in solid-state lasers. Noble gas flash lamps such as Xe, Kr, or mixtures of Xe and Kr have been used for many years. Other flash lamp or electrical discharge or filament sources include Ce lamps, Na lamps, and tungsten filament lamps. For gas lasers, electrical discharges are widely used to excite electrons from a ground state to an excited state or from an excited state to a higher excited state. A key point with electrical discharge gas lasers is that little selectivity is achieved since all gas energy levels accessible via the discharge plasma electrons are populated to a certain degree. Because energy levels other than those directly involved in the lasing process are populated, a significant amount of waste heat is generated and overall laser efficiency suffers. In contrast, optical pumping with monochromatic or narrow bandwidth sources such as semiconductor diode lasers allows only a selected energy level or band to be populated, minimizing heating losses.
The primary reason why gas lasers have not benefitted from the advent of high-power semiconductor diode sources is that until recently such sources were not available at a wavelength suitable for pumping important gas laser transitions. GaAlAs laser diodes and laser diode arrays are currently available with output between about 780-860 nm, while InGaAs laser diodes can be obtained in the range of 940-1100 nm. In recent years AlGalnP diode lasers with output in the visible region (630-690 nm) have begun to be offered commercially, and more recently InGaAsP diode lasers have been offered commercially with substantial output power in the 1.5-1.6 .mu.m and 1.8-1.96 .mu.m ranges, the latter representing the longest wavelength laser diode commercially available. Substantial progress has been reported recently in producing short wavelength blue laser diodes with output close to 400 nm, while newer quantum cascade semiconductor diode laser technology under development can provide wide spectral coverage in the MIR region from 3-15 .mu.m.